In optical disc technologies, data can be read out from a rotating optical disc by irradiating the disc with a relatively weak light beam with a constant intensity, and detecting the light that has been modulated by, and reflected from, the optical disc.
On a read-only optical disc, information is already stored as pits that are arranged spirally during the manufacturing process of the optical disc. On the other hand, on a rewritable optical disc, a recording material film, from/on which data can be read and written optically, is deposited by evaporation process, for example, on the surface of a substrate on which tracks with spiral lands or grooves are arranged. In writing data on a rewritable optical disc, data is written there by irradiating the optical disc with a light beam, of which the optical power has been changed according to the data to be written, and locally changing the property of the recording material film.
It should be noted that the depth of the pits and tracks and the thickness of the recording material film are both smaller than the thickness of the optical disc substrate. For that reason, those portions of the optical disc, where data is stored, define a two-dimensional plane, which is sometimes called a “storage plane” or an “information plane”. However, considering that such a plane actually has a physical dimension in the depth direction, too, the term “storage plane (or information plane)” will be replaced herein by another term “information layer”. Every optical disc has at least one such information layer. Optionally, a single information layer may actually include a plurality of layers such as a phase-change material layer and a reflective layer.
To read data that is stored on a rewritable optical disc or to write data on such an optical disc, the light beam always needs to maintain a predetermined converging state on a target track on an information layer. For that purpose, a “focus control” and a “tracking control” need to be done. The “focus control” means controlling the position of an objective lens along a normal to the surface of the information plane (such a direction will sometimes be referred to herein as “substrate depth direction”) such that the focal point (or at least the converging point) of the light beam is always located on the information layer. On the other hand, the “tracking control” means controlling the position of the objective lens along the radius of a given optical disc (which direction will be referred to herein as a “disc radial direction”) such that the light beam spot is always located right on a target track.
In order to perform such a focus control or a tracking control, the focus error or the tracking error needs to be detected based on the light that has been reflected from the optical disc and the position of the light beam spot needs to be adjusted so as to reduce the error as much as possible. The magnitudes of the focus error and the tracking error are represented by a “focus error (FE) signal” and a “tracking error (TE) signal”, both of which are generated based on the light that has been reflected from the optical disc.
Dual-layer optical discs, in which two information layers are stacked on upon the other, have already been put on the market recently. And now, so-called “multilayer optical discs”, including a stack of three or more information layers, are also being developed. In the following description, however, an optical disc in which N layers (where N is an integer that is equal to or greater than two) are stacked one upon the other (i.e., any optical disc with at least two layers) will be referred to herein as a “multilayer optical disc”.
When data is being read from, or written on, a target one of the information layers of a multilayer optical disc, the optical disc drive needs to set the focus position of the light beam on the target information layer and form a tiny light beam spot on that information layer. As a single multilayer optical disc has multiple information layers, the positions of the light beam spot (i.e., its depths as measured from the disc substrate) need to be detected for the respective information layers in order to set the focus position of the light beam on the target information layer.
The light beam is emitted from a light source that is built in the optical pickup of the optical disc drive. The optical pickup includes an objective lens for converging the light beam that has been emitted from the light source and an actuator for moving that objective lens. The actuator works to bring the objective lens toward, or take it away from, the optical disc. In this manner, the optical disc drive can move the focus position of the light beam (or the position of the light beam spot) with respect to the optical disc, choose a target one of the information layers, and then form a light beam spot on that target information layer. The position of each information layer in the optical disc rotating continues to vary with respect to the optical pickup. However, by getting the focus control done as described above, the light beam spot can always keep up with the target information layer.
Patent Documents Nos. 1 to 4 disclose a technique for properly recognizing multiple information layers by writing layer-by-layer recognition information (e.g., layer numbers) on the respective information layers.